AP Chemistry Midterm Exam
We place 2.0 mol of PCl₅ in a 5.0 L flask and let the system come to equilibrium. Letting X represent the equilibrium concentration of Cl₂, which of the following expressions will find the equilibrium concentration of PCl₃? PCl₅(g)↔PCl₃(g)+Cl₂(g) Kc=0.5
a) X²=[(0.50)(0.40-X)] .5=x²/.4-x x²=.5(.4-x)
We have the following equilibrium: PCl₅(g)↔PCl₃(g)+Cl₂(g) Kc=0.50 What will happen to [CL₂] if additional PCl₅ is added to the flask?
a) it will increase -→ more Cl₂ formed
Consider the chemical system CO+Cl₂↔COCl₂;K=4.6*10⁹ L/mol How do the equilibrium concentrations of the reactants compare to the equilibrium concentration of the product?
a) they are much smaller - large k, product favored
The formation of ClF₃(g) is exothermic. Cl₂(g)+3 F₂(g)↔2 ClF₃(g)+heat. 1. Add Cl₂ 2. Add ClF₃ 3. Raise the temperature 4. Decrease the volume of the flask containing the reaction
1) d→, no ∆ 2) b←, no ∆ 3) a←, k↓ 4) d, shifts to side w/ fewer moles of gas→, no ∆
CO(g)+2 H₂(g)↔CH₃OH(g) ∆H<0 The synthesis of CH₃OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5. Which of the following explains the effect on the equilibrium constant, Kc, when the temperature of the reaction system is increased to 650 K?
c) Kc, will decrease because the reaction is exothermic -shift left K↓
Which of the following correctly relates the two equilibrium constants for the two reactions shown? A+B↔2 C K₁ C↔1/2 A+1/2 B K₂
a) K₂=1/(K₁)^1/2 -reverse reciprocal 1/2 coeff.
Which of the following compounds contains the greatest percent by mass of nitrogen?
a) NH₃ -nitrogen/total*100 14/17
The reaction, A+B→2 C, has the following rate expression: rate=k[A]²[B]. What are the units for the rate constant for this rate law?
c) M²hours¹ -overall 3rd order M¹⁻³/hours M⁻²∙hours⁻¹
Which of the following is a correct statement about reaction order?
c) reaction order can be determined only by experiment -the only one that's true
Which of the following statements is true about bond energies in this reaction?
c) the energy absorbed as the bonds in the reactants are broken in less than the energy released as the bonds in the product are formed -describes exothermic reaction
Which of the following is true for a system whose equilibrium constant is relatively small?
c) the equilibrium lies to the left. -small k, reactant favored.
How many calcium atoms are present in a 40.08 mg sample of solid calcium metal?
a) 6.022*10²⁰ -40.08 mg*1g/1000 mg*1 mol/40.08g*6∙10²³/1 mol=6.02*10²⁰
If equal masses of the following compounds undergo complete combustion, which will yield the greatest mass of CO₂?
a) Benzene, C₆H₆ -greatest % C
In a chemical reaction with first order kinetics, which is true at constant temperature?
a) Half-life and k are both constant -k is only temperature dependent. Temp is constant so k also is, so half life is constant.
Which of the following is the correct form of the equilibrium constant expression for the reaction below? Zn(OH)₂(s)↔Zn²⁺(aq)+2 OH⁻¹(aq)
a) K=[Zn²⁺][OH¹⁻]² -don't include solids!
We place 10 mol of N₂O₄(g) in a 2 L flask at 200°C. After reaching equilibrium, [N₂O₄]=2 M. What is the Kc for the following reaction? N₂O₄(g)↔2 NO₂(g)
b) 18.0 -Kc=6²/2=18
Methanol (CH₃OH) can react with oxygen to produce formaldehyde (H₂CO) and water. How much formaldehyde can be produced by reacting 4.0 moles of methanol with 4.0 moles of oxygen gas?
b) 4.0 moles -4*2/2=4 mol
Which of the following is/are true about a chemical system at equilibrium?
b) III only -true
According to the information in the table above, a 1.00 g sample of which of the following contains the greatest mass of oxygen?
b) MgO -smallest MM=greatest mass
2 NO₂(g)+F₂(g)→2 NO₂F(g) The rate law for the reaction represented by the equation above is rate=k [NO₂][F₂]. Which of the following could be the first elementary step of a two-step mechanism for the reaction if the first step is slow and the second step is fast?
b) NO₂(g)+F₂(g)→NO₂F(g)+F(g) -rate=k[NO₂][F₂] match!
A compound is analyzed and found to contain 1.30 moles of Na, 0.65 moles of Te, and 2.60 moles of O. What is the empirical formula for this compound?
b) Na₂TeO₄ -1.3/.65=2 mol Te .65/.65=1 mol Na 2.6/.05= 4 mol O
The oxygen atom-transfer reaction, CO(g)+NO₂(g)→CO₂(g)+NO(g) follows the rate expression: rate=k[CO][NO₂]. What is the order of reaction with respect to CO?
b) first-order -1st order
A mixture of CO(g) and H₂(g) is pumped into a previously evacuated 2.0 L reaction vessel. The total pressure of the reaction system is 1.2 atm at equilibrium. What will be the total pressure of the system if the volume of the reaction vessel is reduced to 1.0 L at constant temperature?
b) greater than 1.2 atm but less than 2.4 atm - 1/2 volume, 2x pressure shifts back to equal.
We have the following equilibrium: N₂O₄(g)↔2 NO₂(g) Hrxn=+150kJ What will happen to [N₂o₄] if the temperature is increased?
b) it will decrease -→ N₂O₄ goes away
H₂(g)+I₂(g)↔2HI(g) At 450°C, 2.0 moles each of H₂(g), I₂(g), and HI(g) are combined in a 1.0 L rigid container. The value of Kc at 450°C is 50. Which of the following will occur as the system moves toward equilibrium?
b) more HI(g) will form -2²/2*2=1 Q<K shifts right→
A 1.0 mol sample of CO(g) and a 1.0 mol sample of H₂(g) are pumped into a rigid, previously evacuated 2.0 L reaction vessel at 483 K. Which of the following is true at equilibrium?
b)[H₂]<[CO] -H2 reacts faster (2 vs 1)
rate=k[X] For the rate law given above, a plot of which of the following is a straight line?
b)in [X] versus time -1st order
Which of the following particulate diagrams best shows the formation of water vapor from hydrogen gas and oxygen gas in a rigid container at 125°C
c) -follows law of cons of mass
To choose a wavelength to analyze the progress of the reaction, a student records the absorbance spectra of both C₂₅H₃₀N₃⁺(aq) and C₂₅H₃₀N₃OH(aq) in the range of 200-800 nm. The two spectra are presented in the graph below. The student wants to use the spectrophotometer to measure [C₂₅H₃₀N₃⁺] with the greatest sensitivity as the reaction progresses. Which of the following indicates the best wavelength setting and explains why it is best?
c) 590 nm, because only the violet form of the molecule will absorb significantly at this wavelength -no colorless peak there
Which of the following is the correct form of the equilibrium constant expression for the reaction below? 2 SO₃(g)↔2 SO₂(g)+O₂(g)
c) K=[SO₂]²[O₂]/[SO₃]² -products/reactants
PCl₃(g)+Cl₂(g)↔PCl₅(g) Kc=6.5 At a certain point in time, a 1.00 L rigid reaction vessel contains 1.5 mol of PCl₃(g), 1.0 mol of Cl₂(g), and 2.5 mol of PCl₅(g). Which of the following describes how the measured pressure in the reaction vessel will change and why it will change that way as the reaction system approaches equilibrium at constant temperature?
c) the pressure will decrease because Q<Kc -2.5/1.5 Q<K shifts right→
A 0.35 g sample of Li(s) is placed in an Erienmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of a flask to collect the hydrogen gas that is generated. Which of the following changes will most likely increase the rate of reaction between Li(s) and water?
c) using a .35 g sample of Li(s) cut into small pieces -rate of rxn ↑ when surface area ↑
Which of the following solutions has the greatest number of ions in solution?
d) 600 mL of 0.2 M AlCl₃ -120*4=480
The graph above shows the number of molecules that have sufficient energy to overcome Ea at a certain temperature for a given reaction. How would adding a catalyst change the appearance of the graph?
d) a catalyst decrease the activation energy
Which of the following best helps explain why an increase in temperature increases the rate of a chemical reaction?
d) at higher temperatures, high-energy collisions happen more frequently -when T↑ molecules are more KE on average and therefore have more collisions
Determine if the following system is at equilibrium, the reactant concentrations are too high, the product concentrations are too high, or if one simply cannot determine which with the information give. N₂(g)+O₂(g)↔2 NO(g) Temperature=570 K P(N₂)= 1.22 atm P(O₂)=0.31 atm P(NO)=3.38 atm
d) cannot be determined -Q vs K, no k to compare to...
If the concentration of the product were to double, what would happen to the equilibrium constant?
d) it would not change its value -∆ in conc. doesn't affect K
2BaO₂(s)↔2 BaO(s)+O₂(g) ∆H°=162 kJ/mol rxn A sealed rigid vessel contains BaO₂(s) in equilibrium with BaO(s) and O₂(g) as represented by the equation above. Which of the following changes will increase the amount of BaO₂(s) in the vessel?
d) lowering the temperature -shifts left←
We are studying the reaction of A→2 B. A plot of 1/[A] vs. time as the reaction proceeds is linear with a slope of 1.87/Msec. What is the rate expression for the reaction?
d) rate=1.87[A]² -second order rate=k[A]²
Given the data for the reaction A+B→C, determine the rate expression for the reaction. [A], M: 0.1,0.2,0.1... [B], M: 0.2,0.2,0.1... Rate of formation of C,M/s: 40,80,40
d) rate=k[A] -1st order
Relatively slow rates of chemical reaction are associated with which of the following?
d) strong bonds in reactant molecules -↑Ea, slow down
A student prepared five solutions of CuSO₄ with different concentrations, and then filled five curvettes, each containing one of the solutions. The curvettes were placed in a spectrophotometer set to the appropriate wavelength for maximum absorbance. The absorbance of each solution was measured and recorded. The student plotted absorbance versus concentration, as shown in figure above. Which of the following is the most likely explanation for the variance of the data point for the 0.6 M CuSO₄ solution?
d) the curvet used for the 0.6 M solution had not been wiped clean before being put in the spectrophotometer -oils could absorb light which cause abs to be higher than expected
Which of the following states is or are true regarding the reaction below? H2(g)+I2(g)↔2 HI(g)
d)I and II -same number of moles
The rate law for a certain reaction is determined to be: Rate=K[A]²[B]
d)The reaction involves the formation of intermediates. -a reaction step can't be molecular so intermediates must form
Which of the following rate expressions is to be expected for the reaction, A+3B→E, given the mechanism below: step 1. A+B→C; slow step step 2. C→D; fast step step 3. D+2B→E; fast step
d)rate=k[A][B] -slow step
In which of the following systems would the number of moles of the substances present at equilibrium NOT be shifted by a change in the volume of the system at a constant temperature?
e)NO(g)+O₃(g)↔NO₂(g)+O₂(g) -same # of moles